The present invention relates to an electroluminescent device and further to a method of manufacturing same. More particularly, the present invention relates to an electroluminescent device in which the light-emitting substance is a semiconductor. Most particularly, the present invention relates to a storable information display screen for use with an electronic information processing device, in which the light-emitting substance is a light emitting polymer (LEP).
Electroluminescent devices are structures which emit light when subject to an applied electric field. The usual model for the physical process in a semiconductor used in this way is through the radiative combination of electron-hole pairs which are injected into the semiconductor from opposite electrodes. Common examples are light-emitting diodes based on GaP and similar III-V semiconductors.
Although these devices are efficient and are widely used, they are limited in size, and are not easily or economically used in large area displays.
Alternative materials which can be prepared over large areas are known, and among these are the inorganic semiconductors. Most effort has been directed to ZnS.
However, the ZnS system has considerable practical drawbacks, primarily poor reliability. The mechanism in ZnS is believed to be one where acceleration of one type of carrier through the semiconductor under a strong electric field causes local excitation of the semiconductor which relaxes through radiative emission.
Among organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electroluminescence.
Nevertheless, the practical difficulty with these materials is, as with ZnS, their poor reliability, together with difficulties in deposition of the organic layers and the current-injecting electrode layers.
Techniques such as sublimitation of the organic material suffer from the disadvantage that the resultant layer is soft, prone to recrystalization, and unable to support high temperature deposition of top-contact layers.
Techniques such as Langmuir-Blodgett film deposition of suitably-modified aromatics suffer from poor film quality, dilution of the active material, and high cost of fabrication.
An electroluminescent device utilizing anthracene is disclosed in U.S. Pat. No. 3,621,321. This device suffers from high power consumption and low luminescence. In an attempt to provide an improved device, U.S. Pat. No. 4,672,265 describes an electroluminescent device having a double layer structure as its luminescent layer. However, the suggested materials for the double layer structure are organic materials which suffer from the disadvantages mentioned above.
Light emitting polymers (LEPs) are expected to revolutionize the monitor and computer display, since LEP based display may be manufactured very thin and collapsible, e.g., rollable or foldable.
U.S. Pat. No. 5,399,502, which is incorporated by reference as if fully set forth herein discloses an electroluminescent device which includes (k) a semiconductor layer in a form of a thin deuse polymer film comprising at least one conjugated polymer; (ii) a first contact layer in contact with a first surface of the semiconductor layer; and (iii) a second contact layer in contact with a second surface of the semiconductor layer.
The polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers, so that on applying an electric field between the first and second contact layers across the semiconductor layer, so as to render the second contact layer positive relative to the first contact layer, charge carriers are injected into the semiconductor layer and radiation is emitted from the semiconductor layer.
U.S. Pat. No. 5,399,502 further discloses a method of manufacturing the above described electroluminescent device which includes steps of depositing a thin layer of a precursor polymer onto a substrate and then heating the precursor polymer to a high temperature to form the conjugated polymer.
The invention disclosed in U.S. Pat. No. 5,399,502 is based on the discovery that semiconductive conjugated polymers can be caused to exhibit electroluminescence by the injection of charge carriers from suitable contact layers.
However, the electroluminescent device described in U.S. Pat. No. 5,399,502 suffers several limitations.
First, being a laminate, the device is expected to be limited in the degree allowed collapsibility, since under high collapse strain, the layers of the laminate are expected to either break or become separated, in either case, functionality will be severely and irreversibly hampered.
Second, the device requires a thick on-luminescent base substrate for mechanical support.
Third, being a laminate, the device is inherently manufactured using e.g., lithography or other complicated procedures, which are, in most cases, to be conducted under special conditions, such as, but not limited to, vacuum, and are therefore cumbersome and cost and effort demanding. Furthermore, the use of lithography limits the maximal size of devices obtainable.
There is thus a widely recognized need for, and it would be highly advantageous to have, an electroluminescent device devoid of the above inherent limitations.
According to the present invention there is provided an electroluminescent device and a method of manufacturing same.
According to further features in preferred embodiments of the invention described below, there is provided an electroluminescent device comprising a close arrangement including a first set of fibers and a second set of fibers being arranged to form a two dimensional close array of junctions between fibers of the first set of fibers and fibers of the second set of fibers, wherein each of the fibers includes a longitudinal conductive element, whereas fibers of at least one of the first and second sets of fibers further include a longitudinal coat of at least one light emitting substance being in intimate contact with the conductive element, the at least one light emitting substance is selected such that on applying an electric field between crossing conductive elements, radiation is emitted from the light emitting substance.
The electric field applied may be of a constant direction or alternatively of an alternating direction, depending on the luminescent material employed.
According to further features in preferred embodiments of the invention described below, there is provided a storable information display screen for use with an electronic information processing device comprising an electroluminescent device as described above.
According to further features in preferred embodiments of the invention described below, there is provided a method of manufacturing an electroluminescent device comprising the steps of (a) providing first set of fibers and a second set of fibers, each of the fibers including a longitudinal conductive element, whereas fibers of at least one of the first and second sets of fibers further include a coat of at least one light emitting substance being in intimate contact with the conductive element; and (b) arranging the fibers of the first and second sets of fibers to form a two dimensional close array of junctions therebetween, the at least one light emitting substance being selected such that on applying an electric field between crossing conductive elements, so as to render one of the crossing conductive elements positive relative to the other, radiation is emitted from the light emitting substance.
According to still further features in the described preferred embodiments the light emitting substance is a semiconductive polymer has a sufficiently low concentration of extrinsic charge carriers, such that when charge carriers are injected into the semiconductor coat from the conductive elements, radiation is emitted from the polymer.
According to still further features in the described preferred embodiments the longitudinal conductive element is a core and the longitudinal cost of at least one light emitting substance coating the core.
According to still further features in the described preferred embodiments fibers of both the first and second sets of fibers include the conductive element and the coat of the light emitting substance.
According to still further features in the described preferred embodiments the at least one light emitting polymer is poly (p-phenylenevinylene) or a derivative thereof.
According to still further features in the described preferred embodiments fibers including the coat of light emitting substance are about 10 micrometer to and 2 centimeters in width.
According to still further features in the described preferred embodiments the conductive element includes a substance selected from the group consisting of metal, metal oxide, alloy, a conductive polymer and combinations thereof.
According to still further features in the described preferred embodiments the substance is selected from the group consisting of aluminum, aluminum oxide, gold, magnesium/silver alloy, indium oxide and combinations thereof.
According to still further features in the described preferred embodiments the conductive element of each of the fibers including the coat of light emitting substance is about 50-99% of the total thickness of the fibers.
According to still further features in the described preferred embodiments the fibers of the first set of fibers and the fibers of the second set of fibers are arranged substantially perpendicularly to one another.
According to still further features in the described preferred embodiments the fibers of the first set of fibers and the fibers of the second set of fibers are interlocked in a woven arrangement.
According to still further features in the described preferred embodiments the device further comprising an implement for applying the electric field between crossing conductive elements.
The present invention successfully addresses the shortcomings of the presently known configurations by providing a fibers based electroluminescent device which is highly collapsible and simple to manufacture.